Since v9 this has changed. Now there is a different server cluster named Demian, which has two PK servers. The other clusters now have no PK servers.
While you adventure in the [PK] server, you'll find that it is akin to the other servers with a few changes. As long as you are outside of a safe location (towns), you can attack other players by holding down on the CTRL button and selecting the player you wish to attack. This works similarly to Dueling, though no consent is required from the attacked player. By defeating them, you gain PK and disposition.
Furthermore, those that choose to play on the PK server get the benefits of x2 EXP and x2 Drops, allowing them to progress through levels and still obtain a similar amount of drops per level that would be found on other non-PK servers.